This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Oxygen, that supports all aerobic life, is abundant in the atmosphere because of its constant regeneration by photosynthetic water oxidation by green plants, algae and cyanobacteria. This light-requiring reaction is catalyzed by a Mn4Ca cluster associated with photosystem II (PS II). Given the role of PS II in maintaining life on the biosphere and the future visions of a renewable energy economy, it is vital to elucidate the structure and mechanism of the Mn4Ca catalyst, which is sometimes referred to as the ?heart? of the water oxidation process. Although, the electronic and geometric structure of the Mn4Ca water-splitting catalyst has been extensively investigated, the precise structure and mechanism of this catalyst has so far eluded all attempts of determination by x-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) and other spectroscopic techniques. The development of new methods of high resolution absorption and emission x-ray spectroscopy, including x-ray Raman spectroscopy, range-extended EXAFS and site-selective X-ray spectroscopy are critical for providing important new information, that will help in elucidating not only the structure of the complex, but also the mechanism of the photosynthetic water oxidation process. The present proposal builds on the past experiments and addresses some new applications of x-ray emission spectroscopy to the Mn complex in PS II as follows: a) X-ray Raman or RIXS spectroscopy that will address the electronic structure of the Mn complex, b) Kbeta emission and site selective spectroscopy that will selectively probe different Mn sites or different oxidation states in the Mn complex, c) High resolution Mn Kalpha detection of EXAFS that will allow us to collect EXAFS beyond the Fe K-edge thus improving resolution and d) Interatomic Mn Kbeta2,5 spectra that will be a probe for the ligand atoms of Mn, especially to ascertain whether chloride is a ligand of Mn in PS II.